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Cr nanostructures
Journal of Alloys and Compounds 477 (2009) 888–891
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Preparation, characterization and growth mechanism study of CdS/Cr nanostructures L.Q. Qian, S.L. Wang, X. Jia, Y.Y. Liu, W.H. Tang ∗ Department of Physics, Center for Optoelectronics Materials and Devices, Zhejiang Sci-Tech University, Xiasha College Park, Hangzhou, Zhejiang 310018, China
a r t i c l e
i n f o
Article history: Received 27 September 2008 Received in revised form 28 October 2008 Accepted 1 November 2008 Available online 20 December 2008 Keywords: CdS/Cr nanostructures Characterization Growth mechanism
a b s t r a c t Dentiform nanorods and tassellike nanowires CdS/Cr nanostructures were synthesized via a simple onestep thermal evaporation of CdS and Cr metal powder in mass scale. The morphology and detailed nanostructures were characterized by using scanning electron microscope, X-ray diffraction, and transmission electron microscope. The dentiform nanorods were fabricated at a low Ar flow rate, while the tassellike nanowires were created at a higher Ar flow rate. It is found that CdS/Cr dentiform nanostructures are the Cr-doped CdS nanostructure, whereas the tassellike nanowires CdS/Cr heterostructures are CdS nanowires attached with Cr particles on their surfaces. The selected area electron diffraction pattern indicates the Cr particle is polycrystalline, attributable to the {1 1 0}, {2 0 0} crystal planes of Cr. The formation of the CdS/Cr tassellike nanowires was initiated by the Au catalyst vapor–liquid–solid mechanism, and vapor–solid process also played a role in the defining the dentiform nanostructures. Photoluminescence measurements show that the tassellike nanowires have three emission bands around 468 nm, 510 nm and 718 nm, which originate from the enwrapped Cr, the band-gap of bulk CdS backbones and the surface defects of the CdS, respectively. © 2008 Elsevier B.V. All rights reserved.
1. Introduction As an important II-VI group semiconductor material, CdS has a band-gap energy of 2.42 eV, and is used for a host of applications in optoelectronics [1], such as nonlinear optics, flat panel displays, light emitting diodes, lasers, and thin film transistors [2–7]. In recently, the researchers have reported that the structure of CdS played an important role on the influence of the unique physical properties of CdS. Thus, the study on the structures of CdS has attracted a great deal of interest from researchers. CdS multipod-based structures, such as flowerlike microstructures, tetrapod-like microrods, and long branched nanowires were selectively prepared by Lee and Shen [8] on no Au sputtered substrates (Si). And they found that the morphologies of the products could be well controlled by adjusting the deposition position. After that, this group also synthesized single-crystal CdS microbelts by a modified thermal evaporation method with an adiabatic layer in the equipment. The formation of microbelt structures is mainly controlled by two factors: the surface energy and the growth kinetics [9]. Soumitra et al. have reported the production of CdS nanostructures with different Ar flow rates and different reaction
∗ Corresponding author. Tel.: +86 571 86843222; fax: +86 571 86843222. E-mail address: [email protected] (W.H. Tang). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.11.007
temperatures. And they proposed that the VLS and VS mechanism played great parts in the preparation of the CdS nanostructures [10,11]. However, the study on these impacts of the doped-CdS nanostructures or the CdS heterostructures is limited. In fact, the doped nanostructures and the heterostructures could enhance emission peaks and they have lower band-gap energy [12,13], which are valuable for fundamental research and making as nanoscale devices. In this work, we have prepared the doped-CdS nanostructures and the Cd/Cr heterostructures with different shapes such as dentiform nanorods, and tassillike nanowires by a simple thermal evaporation route. The effect of synthesis temperature and the flow rate of Ar gas on the size and shapes of doped nanostructures and heterostructures are studied. 2. Experimental 2.1. The preparation and characterization of precursor All chemical reagents were of analytical grade. The CdS nanopowder, used as the precursor, was prepared by hydrothermal approach. In short, 0.5 M Cd(NO3 )2 and 0.5 M Na2 S were transferred into Teflon lined stainless steel autoclave in same volume, sealed and maintained at 130 ◦ C for 10 h. And then, the resulting yellow solid product was centrifuged, washed with distilled water and ethanol to remove the ions possibly remaining in the final products and finally dried at 80 ◦ C in vacuum for 4 h. All the diffraction peaks of the product shown in Figure 1 can be indexed to cubic CdS with lattice constants a = 5.818 Å, which matches with JCPDS card no.10-0454.
L.Q. Qian et al. / Journal of Alloys and Compounds 477 (2009) 888–891
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(Edinburgh instruments) fluorescence spectrometer with a Xe lamp. The typical selected area electron diffraction (SAED) patterns of the CdS/Cr nanostructures were recorded and the growth mechanism was also discussed.
3. Results and discussion 3.1. Morphology of the CdS/Cr nanostructures
Fig. 1. XRD pattern of the CdS powder precursor.
2.2. The preparation of CdS/Cr nanostructures The appropriate amount of CdS and Cr powders were loaded into a quartz boat. The Si substrates were then clipped over the boat with Au-coated surface facing the CdS and Cr powder. After evacuating the furnace, Ar gas was flown through it and was maintained for the entire deposition period. Then, the furnace was promptly heated up to 900 ◦ C and held for 40 min before it was cooled down to room temperature. By changing the flow rate of Ar gas, we obtained different CdS/Cr morphologies on the Si wafers. The products were characterized by X-ray diffractometer (XRD, Bruker AXS, D8 Discover) with Cu K␣ radiation (=1.5406 Å) at a scanning rate of 10◦ min−1 . The compositional analysis was done by energy-dispersed X-ray spectroscopy (EDS, S-4700). The morphologies of products were characterized by scanning electron microscopy (SEM, JSM-5610, S-4700) and transmission electron microscopy (TEM, TECNAI-12, Philip Apparatus Co., The Netherlands). Photoluminescence (PL) studies of the tassellike nanowires were performed at room temperature with FLSP920
Figure 2 shows the SEM image of the product obtained at 900 ◦ C, when the flow rate of Ar gas was 200 standard-state cubic centimeter per minute (sccm), and dentiform nanorods were found on the Si substrate. Diameter of the Cr-doped CdS nanorods was ∼500 nm, SEM image shows that the pod is irregular quadrangle. As for the interface of CdS, each tetrahedron has a corner in the [0 0 1] direction, which favors the growth of CdS nanorods along the [0 0 1] axis. So the CdS nuclei will grow into rod-shaped structures along c axis. [8] (Figure 2). Figure 3a shows the SEM image of the tassellike nannowires which were synthesized with the flow rate of Ar 350 sccm. The substrate was much closer to the CdS and Cr mixed powder, so the growth temperature was higher than the dentiform nanorods condition and the powder vapor concentration was lower too. The width of these nanowire backbones is uniform and about 500 nm, while the length is ∼50 m. The TEM image reveals that the backbones of the nanowires are homogeneous, and the tassellike parts are assembled out-of-order. Figure 3b inset shows the SEAD pattern of the tassellike parts, and it reveals that they are polycrystalline, attributable to the {1 1 0}, {2 0 0} crystal planes of Cr. 3.2. Structural and compositional studies Figure 4a shows the XRD pattern of the typical Cr-doped CdS nanorods. All the peaks correspond to the hexagonal wurtzite phase of CdS with lattice constants a = 4.134 Å and c =4.718 Å matched with those in the JCPDS data (no.41-1049), which have a little excursion.
Fig. 2. SEM image of the Cr-doped CdS nanorods synthesized at 900 ◦ C with the flow rate of Ar gas 200 sccm.
Fig. 3. (a) SEM image of the CdS/Cr tassellike nanowires synthesized at 900 ◦ C with the flow rate of Ar gas 350 sccm, (b) TEM image of the CdS/Cr tassellike nanowires. The inset shows the SAED pattern of the tassellike parts.
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Fig. 5. XRD pattern of the CdS/Cr tassellike nanowires.
metal was created. In another part, as a light element, the EDS analyzation of S element could be imprecise. From the XRD pattern for nanorods samples, no peaks indexing to Cr is found, and the diffraction peaks indexing to CdS have a small shift. That Cr has been doped into CdS lattice. The EDS spectrum of nanorods shows the element of Cr, which proves again that the Cr was doped into the CdS. An XRD pattern of the tassellike nanowires is shown in Figure 5, which shows that the sample is a mixture of wurtzite CdS and Cr. All peaks in this pattern can be indexed to known wurtzite structure of CdS (JCPDS no. 41-1049) and Cr (JCPDS no. 06-0694), with the selected area electron diffraction pattern shown in Figure 3(b) inset, which reveals that Cr dose not dope in CdS, but enwrapped outside to create heterostructure of CdS-Cr. Fig. 4. (a) XRD pattern of the typical Cr-doped CdS nanorods, (b) EDS spectrum recorded on a bunch of the nanorods.
The chemical composition and the stoichiometry of the doped-CdS nanostructures were investigated through EDS. Figure 4b shows the EDS spectrum recorded on a bunch of the nanorods, which reveals the chemical composition of the nanorods: Cd, S and Cr. And mole proportion of CdS, Cd and Cr could be calculated, which is 2:1:1.2. Because the reaction temperature was high (900 ◦ C), it led to CdS decompound to Cd and S. And then S was sublimated and the Cd
3.3. Growth mechanism Two well-established routes for the growth of nanoribbons, nanorods and nanowires are the catalyst assisted vapor–solid (VS) [14–16] and vapor–liquid–solid (VLS) [17,18] methods. In the VS method, the source materials vaporize to the molecular level with stoichiometric cation–anion molecules, which condensed to the substrate and the molecule will be arranged in such a way that the proper local charge balance and the structural symmetry are
Fig. 6. Schematic view of the formation of different CdS/Cr nanoforms.
L.Q. Qian et al. / Journal of Alloys and Compounds 477 (2009) 888–891
891
CdS/Cr nanowires in Figure 7 shows the room temperature PL spectrum recorded over the CdS/Cr tassellike nanowires with 400 nm excitation. The luminescence properties of pure CdS nanostructures materials have been widely studied [6,5,19–21]. They identified that the green emission band was an intrinsic characteristic of CdS and the infrared emission band was due to trap-state emission related to surface defects of the CdS. In our work, the energy of the 510 nm band agrees with the band-gap of bulk CdS (2.42 eV at room temperature); the one at 718 nm can be identified as the surface defects of the CdS such as S-vacancies. However, the PL band at 468 nm can be attributed to the enwrapped Cr. 4. Conclusion
Fig. 7. PL spectrum of the CdS/Cr tassellike nanowires. The excitation wavelength is 400 nm.
maintained, resulting in a nucleation center. With further intake of the molecules, the surfaces that have lower energy, for example, side surfaces, start to form and the low energy surface tend to be flat. On the other hand, in the VLS process, a thin layer of metal catalyst such as Au is deposited on the substrate, which at high temperature breaks up to form liquid nanodroplets. These metal droplets absorb the incoming source of molecular vapor, and finally, upon supersaturation, solid nanostructures start appearing with the metal nanoparticles at their tips. Thus, the key characteristic of the VLS growth process is the existence of the metal nanoparticles at the tips of the nanostructures. In our case, we believe that for the tassellike nanowires array the VLS process whereas the dentiform nanorods array the VLS and VS process. This growth mechanism was reported by Soumitra et al. [11], which discussed about the growth of the types of networklike, pearlnecklace-like, and helicallike nanowires. On the basis of our SEM and TEM observation and the existing growth models, we explain the formation of different 1-D CdS/Cr nanoforms, and the growth mechanism is presented simply with a schematic diagram shown in Figure 6. At low CdS/Cr vapor concentrations, that is, at high temperature, CdS/Cr powder deposited slowly to form the nanowires in the high kinetic state, and nanowires were initially formed by the VLS technique [11]. However, when the temperature was relatively low, the mobility of the newly arrived molecules was also slow and the density of Cr powder was also larger; as a result, the Cr species deposited on the side wall of the nanowires as another growth point and the newly arrived molecules deposited continually and irregularly. So the tassellike nanowires were formed. In another, at high CdS/Cr vapor concentrations, that is, at low temperature, CdS/Cr vapor species were not only absorbed by Au liquid droplets but also deposited below the Au nanodroplets which were still in their restructuring stage, that is, in the high energy state. Therefore, nanorods were increased and by the influence of VLS process originated, and when the secondary nucleation center of the nanorods was created (Figure 2), nanorods were originated via VS process to create network nanorods. The other formations of the nanostructures are not explained here and should be investigated ulteriorly. 3.4. Photoluminescence properties The optical properties of CdS/Cr tassellike nanowires were investigated by PL measurements at room temperature with a view to further assess their quality. PL spectrum of the heterostructured
Doped CdS nanostructures and heterostructures with different shapes have been fabricated. Morphological control of the CdS/Cr nanostructures was achieved by varying the flow rate of Ar gas. The dentiform nanorods were fabricated at a low Ar flow rate, while the tassellike nanowires were created at a higher Ar flow rate relatively. The results show that CdS/Cr dentiform nanostructures are of hexagonal phase and the heterostructures are CdS nanowires attached with Cr on the surface. The selected area electron diffraction pattern indicates the surface of nanowires is polycrystalline, attributable to the {1 1 0}, {2 0 0} crystal planes of Cr. The formation of the CdS/Cr nanoforms was initiated by the Au catalyst vapor–liquid–solid mechanism, and vapor–solid process also played a role in the defining the dentiform nanostructures. Photoluminescence measurements show that the tassellike nanowires have three emission bands aroud 468 nm, 510 nm and 718 nm, which originate from the enwrapped Cr, the band-gap of bulk CdS backbones and the surface defects of the CdS, respectively. Acknowledgement This work was supported by the National Nature Science Foundation of China (grant no. 50672088 and 60571029). References [1] A. Pan, R. Liu, Q. Yang, Y.C. Zhu, G.Z. Yang, B.S. Zou, K.Q. Chen, J. Phys. Chem. B 109 (2005) 24268–24272. [2] X.F. Duan, C. Niu, V. Sahi, J. Chen, J.W. Parce, S. Empedocles, J.L. Goldman, Nature 425 (2003) 274–278. [3] Y.K. Liu, J.A. Zapien, C.Y. Geng, Y.Y. Shan, C.S. Lee, S.T. Lee, Appl. Phys. Lett. 85 (2004) 3241–3243. [4] J. Zhang, F. Jiang, L. Zhang, J. Phys. Chem. B 108 (2004) 7002–7005. [5] M. Agata, H. Kurase, S. Hayashi, K. Yamamoto, Solid State Commun. 76 (1990) 1061–1065. [6] B. Ullrich, D.M. Bagnall, H. Sakai, Y. Segawa, Solid State Commun. 109 (1999) 757–760. [7] M.V. Artemyev, V. Sperling, U. Woggon, J. Appl. Phys. 81 (1997) 6975–6977. [8] G.Z. Shen, C.J. Lee, Crystal Growth Design 5 (2005) 1085–1089. [9] G.Z. Shen, J.H. Cho, J.K. Yoo, G.C. Yi, C.J. Lee, J. Phys. Chem. B 109 (2005) 9294–9298. [10] K. Soumitra, B. Satpati, P.V. Satyam, S. Chaudhuri, J. Phys. Chem. B 109 (2005) 19134–19138. [11] K. Soumitra, C. Subhadra, J. Phys. Chem. B 110 (2006) 4542–4547. [12] S.M. Zhou, Mater. Lett. 61 (2007) 119–122. [13] W.H. Tang, X.Li. Fu, Z.Y. Zhang, L.H. Li, Chinese Phys. 15 (2006) 773–777. [14] G.W. Sears, Acta Metall. 3 (1955) 361–367. [15] P. Yang, C.M. Lieber, J. Mater. Res. 12 (1997) 2981–2996. [16] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947–1949. [17] R.S. Wagner, W.C. Ellis, Appl. Phys. Lett. 4 (1964) 89–90. [18] Y. Wu, P. Yang, J. Am. Chem. Soc. 123 (2001) 3165–3166. [19] J. Butty, N. Peyghambarian, Y.H. Kao, J.D. Mackenzie, Appl. Phys. Lett. 69 (1996) 3224–3226. [20] M.S. Gudiksen, C.M. Lieber, J. Am. Chem. Soc. 122 (2000) 8801–8802. [21] W.F. Liu, C.G. Jin, C. Jia, L.Z. Yao, W.L. Cai, X.G. Li, Chem. Lett. 33 (2004) 228– 229.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Preparation, characterization and growth mechanism study of CdS/Cr nanostructures L.Q. Qian, S.L. Wang, X. Jia, Y.Y. Liu, W.H. Tang ∗ Department of Physics, Center for Optoelectronics Materials and Devices, Zhejiang Sci-Tech University, Xiasha College Park, Hangzhou, Zhejiang 310018, China
a r t i c l e
i n f o
Article history: Received 27 September 2008 Received in revised form 28 October 2008 Accepted 1 November 2008 Available online 20 December 2008 Keywords: CdS/Cr nanostructures Characterization Growth mechanism
a b s t r a c t Dentiform nanorods and tassellike nanowires CdS/Cr nanostructures were synthesized via a simple onestep thermal evaporation of CdS and Cr metal powder in mass scale. The morphology and detailed nanostructures were characterized by using scanning electron microscope, X-ray diffraction, and transmission electron microscope. The dentiform nanorods were fabricated at a low Ar flow rate, while the tassellike nanowires were created at a higher Ar flow rate. It is found that CdS/Cr dentiform nanostructures are the Cr-doped CdS nanostructure, whereas the tassellike nanowires CdS/Cr heterostructures are CdS nanowires attached with Cr particles on their surfaces. The selected area electron diffraction pattern indicates the Cr particle is polycrystalline, attributable to the {1 1 0}, {2 0 0} crystal planes of Cr. The formation of the CdS/Cr tassellike nanowires was initiated by the Au catalyst vapor–liquid–solid mechanism, and vapor–solid process also played a role in the defining the dentiform nanostructures. Photoluminescence measurements show that the tassellike nanowires have three emission bands around 468 nm, 510 nm and 718 nm, which originate from the enwrapped Cr, the band-gap of bulk CdS backbones and the surface defects of the CdS, respectively. © 2008 Elsevier B.V. All rights reserved.
1. Introduction As an important II-VI group semiconductor material, CdS has a band-gap energy of 2.42 eV, and is used for a host of applications in optoelectronics [1], such as nonlinear optics, flat panel displays, light emitting diodes, lasers, and thin film transistors [2–7]. In recently, the researchers have reported that the structure of CdS played an important role on the influence of the unique physical properties of CdS. Thus, the study on the structures of CdS has attracted a great deal of interest from researchers. CdS multipod-based structures, such as flowerlike microstructures, tetrapod-like microrods, and long branched nanowires were selectively prepared by Lee and Shen [8] on no Au sputtered substrates (Si). And they found that the morphologies of the products could be well controlled by adjusting the deposition position. After that, this group also synthesized single-crystal CdS microbelts by a modified thermal evaporation method with an adiabatic layer in the equipment. The formation of microbelt structures is mainly controlled by two factors: the surface energy and the growth kinetics [9]. Soumitra et al. have reported the production of CdS nanostructures with different Ar flow rates and different reaction
∗ Corresponding author. Tel.: +86 571 86843222; fax: +86 571 86843222. E-mail address: [email protected] (W.H. Tang). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.11.007
temperatures. And they proposed that the VLS and VS mechanism played great parts in the preparation of the CdS nanostructures [10,11]. However, the study on these impacts of the doped-CdS nanostructures or the CdS heterostructures is limited. In fact, the doped nanostructures and the heterostructures could enhance emission peaks and they have lower band-gap energy [12,13], which are valuable for fundamental research and making as nanoscale devices. In this work, we have prepared the doped-CdS nanostructures and the Cd/Cr heterostructures with different shapes such as dentiform nanorods, and tassillike nanowires by a simple thermal evaporation route. The effect of synthesis temperature and the flow rate of Ar gas on the size and shapes of doped nanostructures and heterostructures are studied. 2. Experimental 2.1. The preparation and characterization of precursor All chemical reagents were of analytical grade. The CdS nanopowder, used as the precursor, was prepared by hydrothermal approach. In short, 0.5 M Cd(NO3 )2 and 0.5 M Na2 S were transferred into Teflon lined stainless steel autoclave in same volume, sealed and maintained at 130 ◦ C for 10 h. And then, the resulting yellow solid product was centrifuged, washed with distilled water and ethanol to remove the ions possibly remaining in the final products and finally dried at 80 ◦ C in vacuum for 4 h. All the diffraction peaks of the product shown in Figure 1 can be indexed to cubic CdS with lattice constants a = 5.818 Å, which matches with JCPDS card no.10-0454.
L.Q. Qian et al. / Journal of Alloys and Compounds 477 (2009) 888–891
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(Edinburgh instruments) fluorescence spectrometer with a Xe lamp. The typical selected area electron diffraction (SAED) patterns of the CdS/Cr nanostructures were recorded and the growth mechanism was also discussed.
3. Results and discussion 3.1. Morphology of the CdS/Cr nanostructures
Fig. 1. XRD pattern of the CdS powder precursor.
2.2. The preparation of CdS/Cr nanostructures The appropriate amount of CdS and Cr powders were loaded into a quartz boat. The Si substrates were then clipped over the boat with Au-coated surface facing the CdS and Cr powder. After evacuating the furnace, Ar gas was flown through it and was maintained for the entire deposition period. Then, the furnace was promptly heated up to 900 ◦ C and held for 40 min before it was cooled down to room temperature. By changing the flow rate of Ar gas, we obtained different CdS/Cr morphologies on the Si wafers. The products were characterized by X-ray diffractometer (XRD, Bruker AXS, D8 Discover) with Cu K␣ radiation (=1.5406 Å) at a scanning rate of 10◦ min−1 . The compositional analysis was done by energy-dispersed X-ray spectroscopy (EDS, S-4700). The morphologies of products were characterized by scanning electron microscopy (SEM, JSM-5610, S-4700) and transmission electron microscopy (TEM, TECNAI-12, Philip Apparatus Co., The Netherlands). Photoluminescence (PL) studies of the tassellike nanowires were performed at room temperature with FLSP920
Figure 2 shows the SEM image of the product obtained at 900 ◦ C, when the flow rate of Ar gas was 200 standard-state cubic centimeter per minute (sccm), and dentiform nanorods were found on the Si substrate. Diameter of the Cr-doped CdS nanorods was ∼500 nm, SEM image shows that the pod is irregular quadrangle. As for the interface of CdS, each tetrahedron has a corner in the [0 0 1] direction, which favors the growth of CdS nanorods along the [0 0 1] axis. So the CdS nuclei will grow into rod-shaped structures along c axis. [8] (Figure 2). Figure 3a shows the SEM image of the tassellike nannowires which were synthesized with the flow rate of Ar 350 sccm. The substrate was much closer to the CdS and Cr mixed powder, so the growth temperature was higher than the dentiform nanorods condition and the powder vapor concentration was lower too. The width of these nanowire backbones is uniform and about 500 nm, while the length is ∼50 m. The TEM image reveals that the backbones of the nanowires are homogeneous, and the tassellike parts are assembled out-of-order. Figure 3b inset shows the SEAD pattern of the tassellike parts, and it reveals that they are polycrystalline, attributable to the {1 1 0}, {2 0 0} crystal planes of Cr. 3.2. Structural and compositional studies Figure 4a shows the XRD pattern of the typical Cr-doped CdS nanorods. All the peaks correspond to the hexagonal wurtzite phase of CdS with lattice constants a = 4.134 Å and c =4.718 Å matched with those in the JCPDS data (no.41-1049), which have a little excursion.
Fig. 2. SEM image of the Cr-doped CdS nanorods synthesized at 900 ◦ C with the flow rate of Ar gas 200 sccm.
Fig. 3. (a) SEM image of the CdS/Cr tassellike nanowires synthesized at 900 ◦ C with the flow rate of Ar gas 350 sccm, (b) TEM image of the CdS/Cr tassellike nanowires. The inset shows the SAED pattern of the tassellike parts.
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Fig. 5. XRD pattern of the CdS/Cr tassellike nanowires.
metal was created. In another part, as a light element, the EDS analyzation of S element could be imprecise. From the XRD pattern for nanorods samples, no peaks indexing to Cr is found, and the diffraction peaks indexing to CdS have a small shift. That Cr has been doped into CdS lattice. The EDS spectrum of nanorods shows the element of Cr, which proves again that the Cr was doped into the CdS. An XRD pattern of the tassellike nanowires is shown in Figure 5, which shows that the sample is a mixture of wurtzite CdS and Cr. All peaks in this pattern can be indexed to known wurtzite structure of CdS (JCPDS no. 41-1049) and Cr (JCPDS no. 06-0694), with the selected area electron diffraction pattern shown in Figure 3(b) inset, which reveals that Cr dose not dope in CdS, but enwrapped outside to create heterostructure of CdS-Cr. Fig. 4. (a) XRD pattern of the typical Cr-doped CdS nanorods, (b) EDS spectrum recorded on a bunch of the nanorods.
The chemical composition and the stoichiometry of the doped-CdS nanostructures were investigated through EDS. Figure 4b shows the EDS spectrum recorded on a bunch of the nanorods, which reveals the chemical composition of the nanorods: Cd, S and Cr. And mole proportion of CdS, Cd and Cr could be calculated, which is 2:1:1.2. Because the reaction temperature was high (900 ◦ C), it led to CdS decompound to Cd and S. And then S was sublimated and the Cd
3.3. Growth mechanism Two well-established routes for the growth of nanoribbons, nanorods and nanowires are the catalyst assisted vapor–solid (VS) [14–16] and vapor–liquid–solid (VLS) [17,18] methods. In the VS method, the source materials vaporize to the molecular level with stoichiometric cation–anion molecules, which condensed to the substrate and the molecule will be arranged in such a way that the proper local charge balance and the structural symmetry are
Fig. 6. Schematic view of the formation of different CdS/Cr nanoforms.
L.Q. Qian et al. / Journal of Alloys and Compounds 477 (2009) 888–891
891
CdS/Cr nanowires in Figure 7 shows the room temperature PL spectrum recorded over the CdS/Cr tassellike nanowires with 400 nm excitation. The luminescence properties of pure CdS nanostructures materials have been widely studied [6,5,19–21]. They identified that the green emission band was an intrinsic characteristic of CdS and the infrared emission band was due to trap-state emission related to surface defects of the CdS. In our work, the energy of the 510 nm band agrees with the band-gap of bulk CdS (2.42 eV at room temperature); the one at 718 nm can be identified as the surface defects of the CdS such as S-vacancies. However, the PL band at 468 nm can be attributed to the enwrapped Cr. 4. Conclusion
Fig. 7. PL spectrum of the CdS/Cr tassellike nanowires. The excitation wavelength is 400 nm.
maintained, resulting in a nucleation center. With further intake of the molecules, the surfaces that have lower energy, for example, side surfaces, start to form and the low energy surface tend to be flat. On the other hand, in the VLS process, a thin layer of metal catalyst such as Au is deposited on the substrate, which at high temperature breaks up to form liquid nanodroplets. These metal droplets absorb the incoming source of molecular vapor, and finally, upon supersaturation, solid nanostructures start appearing with the metal nanoparticles at their tips. Thus, the key characteristic of the VLS growth process is the existence of the metal nanoparticles at the tips of the nanostructures. In our case, we believe that for the tassellike nanowires array the VLS process whereas the dentiform nanorods array the VLS and VS process. This growth mechanism was reported by Soumitra et al. [11], which discussed about the growth of the types of networklike, pearlnecklace-like, and helicallike nanowires. On the basis of our SEM and TEM observation and the existing growth models, we explain the formation of different 1-D CdS/Cr nanoforms, and the growth mechanism is presented simply with a schematic diagram shown in Figure 6. At low CdS/Cr vapor concentrations, that is, at high temperature, CdS/Cr powder deposited slowly to form the nanowires in the high kinetic state, and nanowires were initially formed by the VLS technique [11]. However, when the temperature was relatively low, the mobility of the newly arrived molecules was also slow and the density of Cr powder was also larger; as a result, the Cr species deposited on the side wall of the nanowires as another growth point and the newly arrived molecules deposited continually and irregularly. So the tassellike nanowires were formed. In another, at high CdS/Cr vapor concentrations, that is, at low temperature, CdS/Cr vapor species were not only absorbed by Au liquid droplets but also deposited below the Au nanodroplets which were still in their restructuring stage, that is, in the high energy state. Therefore, nanorods were increased and by the influence of VLS process originated, and when the secondary nucleation center of the nanorods was created (Figure 2), nanorods were originated via VS process to create network nanorods. The other formations of the nanostructures are not explained here and should be investigated ulteriorly. 3.4. Photoluminescence properties The optical properties of CdS/Cr tassellike nanowires were investigated by PL measurements at room temperature with a view to further assess their quality. PL spectrum of the heterostructured
Doped CdS nanostructures and heterostructures with different shapes have been fabricated. Morphological control of the CdS/Cr nanostructures was achieved by varying the flow rate of Ar gas. The dentiform nanorods were fabricated at a low Ar flow rate, while the tassellike nanowires were created at a higher Ar flow rate relatively. The results show that CdS/Cr dentiform nanostructures are of hexagonal phase and the heterostructures are CdS nanowires attached with Cr on the surface. The selected area electron diffraction pattern indicates the surface of nanowires is polycrystalline, attributable to the {1 1 0}, {2 0 0} crystal planes of Cr. The formation of the CdS/Cr nanoforms was initiated by the Au catalyst vapor–liquid–solid mechanism, and vapor–solid process also played a role in the defining the dentiform nanostructures. Photoluminescence measurements show that the tassellike nanowires have three emission bands aroud 468 nm, 510 nm and 718 nm, which originate from the enwrapped Cr, the band-gap of bulk CdS backbones and the surface defects of the CdS, respectively. Acknowledgement This work was supported by the National Nature Science Foundation of China (grant no. 50672088 and 60571029). References [1] A. Pan, R. Liu, Q. Yang, Y.C. Zhu, G.Z. Yang, B.S. Zou, K.Q. Chen, J. Phys. Chem. B 109 (2005) 24268–24272. [2] X.F. Duan, C. Niu, V. Sahi, J. Chen, J.W. Parce, S. Empedocles, J.L. Goldman, Nature 425 (2003) 274–278. [3] Y.K. Liu, J.A. Zapien, C.Y. Geng, Y.Y. Shan, C.S. Lee, S.T. Lee, Appl. Phys. Lett. 85 (2004) 3241–3243. [4] J. Zhang, F. Jiang, L. Zhang, J. Phys. Chem. B 108 (2004) 7002–7005. [5] M. Agata, H. Kurase, S. Hayashi, K. Yamamoto, Solid State Commun. 76 (1990) 1061–1065. [6] B. Ullrich, D.M. Bagnall, H. Sakai, Y. Segawa, Solid State Commun. 109 (1999) 757–760. [7] M.V. Artemyev, V. Sperling, U. Woggon, J. Appl. Phys. 81 (1997) 6975–6977. [8] G.Z. Shen, C.J. Lee, Crystal Growth Design 5 (2005) 1085–1089. [9] G.Z. Shen, J.H. Cho, J.K. Yoo, G.C. Yi, C.J. Lee, J. Phys. Chem. B 109 (2005) 9294–9298. [10] K. Soumitra, B. Satpati, P.V. Satyam, S. Chaudhuri, J. Phys. Chem. B 109 (2005) 19134–19138. [11] K. Soumitra, C. Subhadra, J. Phys. Chem. B 110 (2006) 4542–4547. [12] S.M. Zhou, Mater. Lett. 61 (2007) 119–122. [13] W.H. Tang, X.Li. Fu, Z.Y. Zhang, L.H. Li, Chinese Phys. 15 (2006) 773–777. [14] G.W. Sears, Acta Metall. 3 (1955) 361–367. [15] P. Yang, C.M. Lieber, J. Mater. Res. 12 (1997) 2981–2996. [16] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947–1949. [17] R.S. Wagner, W.C. Ellis, Appl. Phys. Lett. 4 (1964) 89–90. [18] Y. Wu, P. Yang, J. Am. Chem. Soc. 123 (2001) 3165–3166. [19] J. Butty, N. Peyghambarian, Y.H. Kao, J.D. Mackenzie, Appl. Phys. Lett. 69 (1996) 3224–3226. [20] M.S. Gudiksen, C.M. Lieber, J. Am. Chem. Soc. 122 (2000) 8801–8802. [21] W.F. Liu, C.G. Jin, C. Jia, L.Z. Yao, W.L. Cai, X.G. Li, Chem. Lett. 33 (2004) 228– 229.